Many types of input devices may be used to provide input to computing devices, such as buttons or keys, mice, trackballs, joysticks, touch screens and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation. Typically touch screens can include a touch sensor panel, which may be a clear panel with a touch-sensitive surface, and a display device that can be positioned behind the panel so that the touch-sensitive surface substantially covers the viewable area of the display device. Touch screens allow a user to provide various types of input to the computing device by touching the touch sensor panel using a finger, stylus, or other object at a location dictated by a user interface being displayed by the display device. In general, touch screens can recognize a touch event and the position of the touch event on the touch sensor panel, and the computing system can then interpret the touch event in accordance with the display appearing at the time of the touch event, and thereafter can perform one or more actions based on the touch event.
Touch sensor panels can be formed from a matrix of row and column traces, with sensors or pixels present where the rows and columns cross over each other while being separated by a dielectric material. Each row can be driven by a stimulation signal, and touch locations can be identified through changes in the stimulation signal. Typically, a touch location is sensed based on an interference of the stimulation signal, such that a touch location may correspond to a location where the stimulation signal is the weakest. Touch sensor panels may generally be configured to detect touches from a user's fingers, which generally have a surface area that contacts the touch sensor panel to disturb the stimulation signal sufficiently for touch location to be recognized.
Touch sensor panels may also incorporate or be co-located with force sensors. For example, an array of force sensors may be placed underneath the touch panel, or the touch panel may be constructed such that the capacitance across the panel varies in response to applied force and the capacitance is detected. Other arrangements of force sensors that translate an applied force into an electrical signal may also be used in a touch sensor panel in order to provide a force sensitive touch sensor panel. Force sensitive touch sensors may generally be configured to detect force applied by a user's fingers, which can apply sufficient force to cause the touch panel to deform. In a mobile device that has a display that is both touch sensitive and force sensitive, force applied to the display may enable additional forms of interaction with the device such as previewing content.
When a user is holding a mobile device and interacting with the device, they may hold the device in such a fashion that their hand or fingers rests on the rear of the device. When they interact with the device, the portion of the hand contacting the rear of the device may apply force to the rear of the device, either unintentionally or intentionally. This force applied to the rear of the device may cause the rear housing to deform, thereby transferring a deformation to the force sensors. Force sensors that detect force by measuring capacitance variation due to deformation or by directly sensing force may not be able to distinguish whether the force is due to force on the front of the device or the rear of the device. The force sensor may then detect the force as if the user had intended to apply it to the front of the device. This results in incorrect detection of force.